Electrical contacts are a ubiquitous feature of electrical apparatus and installations and are present, for instance, in such components as switches, plugs, relays, and commutators. Electrical contacts can be designed to carry very small currents, of the order of milliamperes or even less, and to operate in circuits having very small open circuit voltages, of the order of a few volts. On the other hand, heavy-duty contacts can carry kiloamperes of current and operate in circuits having perhaps many kilovolts of open circuit voltage. The technologies involved under these two sets of conditions are quite different, and since this application deals with light-duty electrical contacts, our discussion will be restricted thereto.
Because light-duty electrical contacts typically are used in relatively low voltage applications, it is important that the voltage drop across the contact be small and, perhaps even more importantly, remain substantially constant with time. This voltage drop is, of course, a manifestation of the nonzero resistance of the contact to the flow of electricity, the so-called contact resistance. The contact resistance is comprised of at least two components, namely, the construction resistance which is, inter alia, due to the typically relatively small actual current carrying area of even an ideally clean contact, and the so-called film resistance due to the presence of a contaminating film on real contacts. Whereas it is difficult to change the former component, the latter can be reduced through application of appropriate measures.
The prior art knows a variety of approaches towards eliminating or reducing film resistance. Probably the most commonly employed approach is the use of noble metal contacts. A typical noble metal contact layer consists of a thin gold-rich electrodeposited alloy film on a base metal structure. Such contacts are generally very reliable, can be made to have good wear properties, and typically have stable and low contact resistance. However, the recent rise and fluctuation in the price of gold has led to a search for contact materials that do not contain gold.
The consumption of gold can be reduced by the use of thinner gold contact layers. However, it has been found that, when gold is deposited in a layer less than about 1 .mu.m thick, it is porous and does not withstand corrosion well. A prior art method of dealing with this shortcoming uses a thin layer of noble metal, e.g., gold, not more than 0.2 to 2 .mu.m thick, in combination with a layer of a mixture comprising hard electrically conducting particles (e.g., ruthenium (Ru)) in a binder of a pasty consistency, e.g., paraffin or vaseline. The particles are to have such hardness that they are capable of penetrating through any local corrosion film on the mating noble metal surface. The minimum diameter of the particles is to be no less than the maximum thickness of the insulating layer covering the contact area, and the number of particles is to be so high that the contact resistance becomes sufficiently low. See Dutch Pat. No. 8,001,555.
Another approach was described by D. J. Pedder, Electric Components Science and Technology, Gordon and Breach Publishers, Ltd., Volume 2, pp. 259-261 (1976). The approach consists in the use of a powder-metallurgically produced composite consisting of ruthenium oxide (RuO.sub.2) particles embedded in a silver matrix. The composite is produced by mixing silver and ruthenium particles in desired proportions, compacting at a pressure of 10 tons per square inch, sintering and oxidizing the ruthenium particles to RuO.sub.2 either simultaneously or sequentially. The composite is then coined to increase the density to a value approaching the theoretical density.
The thus produced contact surfaces contain RuO.sub.2 particles both in, and protruding from, the silver surface. Contacts formed from this composite were tested and found to have low and relatively stable resistance when used in a corrosive atmosphere. In explanation of this observation, Pedder suggests that small islands of a nontarnishing conducting material provide conducting paths through an otherwise tarnished surface. It is also suggested that the oxide particles, being harder than the corrosion layer formed on the silver surfaces, may rupture such films on the opposing contact surface.
The above prior art method substantially relies on the volume increase attendant the formation of RuO.sub.2 from Ru to result in the projection of some particles above the surface of the silver matrix. It is thus restricted with regard to possible matrix materials and embedded particle material. The method is also typically restricted to relatively low concentrations of RuO.sub.2 particles, due to, inter alia, the deformation and volume change of the composite material resulting from the volume change of the embedded particles. Furthermore, the method permits only limited control of surface roughness. See also U.S. Pat. No. 3,778,257 issued Dec. 11, 1973 to T. A. Davies, for "Light-Duty Electrical Contact of Silver and Ruthenium Oxide".
It thus appears that a method for producing composite light-duty contact material that is applicable to a large group of matrix materials, including base metals such as copper, that permits control of the resulting surface condition of the composite, and that typically yields dimensionally stable parts is not taught by the prior art, although such a method would be of substantial economic and technological interest.